This invention relates generally to a novel semiconductor device which is adapted to perform the operation of a thyristor and to be capable of controlling a large current by switching of a small current, and more particularly is directed to a semiconductor device in which a thyristor section and a transistor section are constructed to be partly common to each other; the transistor section is supplied with a main current; and the thyristor section is supplied with one portion of the current to perform a switching operation to thereby control the transistor section.